Re: [PATCH v8 5/8] mm: Device exclusive memory access

[Jason Gunthorpe]

On Tue, May 18, 2021 at 07:45:05PM -0400, Peter Xu wrote:
> On Tue, May 18, 2021 at 08:03:27PM -0300, Jason Gunthorpe wrote:
> > Logically during fork all these device exclusive pages should be
> > reverted back to their CPU pages, write protected and the CPU page PTE
> > copied to the fork.
> > 
> > We should not copy the device exclusive page PTE to the fork. I think
> > I pointed to this on an earlier rev..
> 
> Agreed.  Though please see the question I posted in the other thread: now I am
> not very sure whether we'll be able to mark a page as device exclusive if that
> page has mapcount>1.

IMHO it is similar to write protect done by filesystems on shared
mappings - all VMAs with a copy of the CPU page have to get switched
to the device exclusive PTE. This is why the rmap stuff is involved in
the migration helpers

Jason